Determine the intervals over which the function is increasing, decreasing, or constant.

f(x) = x - 3x

Okay, so I get a line with one slope and it's decreasing... The correct answer is:
Decreasing: (-8, 1.5) Increasing: (1.5, 8)

HOW is that possible?? Lol. Please I need your help here because I am lost!

f(x) = x^2 - 3x
f'(x) = 2x - 3 = 0 iff
x = 3/2

Thus critical value is 3/2.

(-inf, 3/2]
test point: 0
f'(0) = 2(0) - 3 < 0
Thus function is decreasing on this interval.

[3/2, inf)
test point: 2
f'(2) = 2(2) - 3 > 0
Thus function is increasing on this interval.

Since your interval ends at -8 and 8, it looks like the domain of your function is restricted (-8, 8).